U.S. ARMY MEDICAL DEPARTMENT CENTER AND SCHOOL FORT SAM HOUSTON, TEXAS PRESERVATION OF FOODS SUBCOURSE MD0703 EDITION 100

Transcription

1 U.S. ARMY MEDICAL DEPARTMENT CENTER AND SCHOOL FORT SAM HOUSTON, TEXAS PRESERVATION OF FOODS SUBCOURSE MD0703 EDITION 100

2 DEVELOPMENT This subcourse is approved for resident and correspondence course instruction. It reflects the current thought of the Academy of Health Sciences and conforms to printed Department of the Army doctrine as closely as currently possible. Development and progress render such doctrine continuously subject to change. ADMINISTRATION For comments or questions regarding enrollment, student records, or shipments, contact the Nonresident Instruction Branch at DSN , commercial (210) , toll-free ; fax: or DSN , or write to: COMMANDER AMEDDC&S ATTN MCCS HSN TH STREET SUITE 4192 FORT SAM HOUSTON TX Approved students whose enrollments remain in good standing may apply to the Nonresident Instruction Branch for subsequent courses by telephone, letter, or . Be sure your social security number is on all correspondence sent to the Academy of Health Sciences. CLARIFICATION OF TRAINING LITERATURE TERMINOLOGY When used in this publication, words such as "he," "him," "his," and "men" are intended to include both the masculine and feminine genders, unless specifically stated otherwise or when obvious in context..

3 TABLE OF CONTENTS Lesson Paragraphs 1 INTRODUCTION TO MICROBIOLOGY Section I. Agents Causing Food Spoilage Section II. Microbial Growth Exercises 2 FOOD MICROBIOLOGY Section I. Microbiology of Dairy Products Section II. Microbiology of Meats Section III. Microbiology of Poultry and Shell Eggs Section IV. Microbiology of Fruits and Vegetables Exercises 3 FOOD PRESERVATION Section I. Introduction to Preservation of Foods Section II. Methods of Preservation Addition of Chemicals Section III. Methods of Preservation--Thermal Methods Section IV. Preservation of Fruits and Vegetables Section V. Food Additives Section VI. Aseptic Storage System for Canning Exercises MD0703 i

4 CORRESPONDENCE COURSE OF THE U.S. ARMY MEDICAL DEPARTMENT CENTER AND SCHOOL SUBCOURSE MD0703 PRESERVATION OF FOODS INTRODUCTION How do bacteria reproduce? Does the bacterial cell contain a nucleus? What are the shapes of bacteria? If you cannot answer these questions now, you should be able to when you have completed this subcourse, and you should also know the answers to many other questions. For those of you who already know this material, let it serve as a review. Why are we interested in bacteria? Because some bacteria are capable of waging war on the human race and some bacteria are capable of benefiting our lives. We need to know the difference. Bacteria are microorganisms and microorganisms are the smallest of all organisms; for example, 2,000 of them can be lined up across the head of a common pin. In this subcourse, we will be concerned with those tiny organisms that are unfriendly, because they are responsible for a large percentage of spoilage in foods. We believe it is important to know about those microorganisms that cause food deterioration so that we can eliminate deterioration in foods before it occurs. Subcourse Components: This subcourse consists of three lessons. Lesson 1, Introduction to Microbiology. Lesson 2, Food Microbiology. Lesson 3, Food Preservation. Credit Awarded: To receive credit hours, you must be officially enrolled and complete an examination furnished by the Nonresident Instruction Branch at Fort Sam Houston, Texas. Upon successful completion of the examination for this subcourse, you will be awarded 8 credit hours. You can enroll by going to the web site and enrolling under "Self Development" (School Code 555). A listing of correspondence courses and subcourses available through the Nonresident Instruction Section is found in Chapter 4 of DA Pamphlet , Army Correspondence Course Program Catalog. The DA PAM is available at the following website: MD0703 ii

5 LESSON ASSIGNMENT LESSON 1 Introduction to Microbiology. LESSON ASSIGNMENT Paragraphs 1-1 through LESSON OBJECTIVES After completing this lesson, you should be able to: 1-1. Identify the definitions of common terms related to microbiology List various parts of the culturing process of microorganisms List five organisms of the Kingdom Protista Identify bacteria morphology and physiology, to include shape, size, arrangement, reproduction, and optimum growth conditions Identify characteristics of microorganisms smaller than bacteria and molds, yeasts, and enzymes List the steps of microbial growth List the four phases of microbial growth and four ways to lengthen the lag phase Identify factors influencing growth of microorganisms, to include nutritional requirements, oxygen requirements, ph requirements, temperature requirements and the effect of heat or cold, and moisture requirements Identify terms used in relation to inhibiting or destroying microorganisms. SUGGESTION After studying the assignment, complete the exercises of this lesson. These exercises will help you to achieve the lesson objectives. MD

6 1-1. INTRODUCTION LESSON 1 INTRODUCTION TO MICROBIOLOGY Section I. AGENTS CAUSING FOOD SPOILAGE a. Early Man and Food. The history of man's civilization can be directly correlated with his knowledge of the science of the preservation and curing of foods. Preserving and curing food is directly related to the microbiology of the particular food. While Early Man had no conclusive evidence of the existence of microorganisms, he did know that food would spoil under certain conditions, and he was able to take certain corrective measures to prevent this spoilage. Man must have a constant source of food in order for civilization to advance. The human race has lived under two different nutritional systems. Under the first system, which terminated about 6000 B.C., man was essentially a food gatherer. He was concerned only with obtaining food to continue to exist and was not able to gather or produce food in sufficient quantities so that there was leisure time for cultural pursuits. About 6000 B.C., man became a food producer. Food then became available in sufficient quantities so that there was sufficient food for periods of leisure and, in fact, certain groups were freed of the necessity of producing food and were able to become scholars. Cereal and livestock farming are essential to the founding of civilization. With the growing of cereals and domestication of livestock, man finally achieved the status of a food producer. b. The Necessity of Preserving Food. Man, as a food producer, was able to settle in one locale and remain there on a permanent basis. The food he produced, of course, was subject to spoilage and he had to become adept at preserving it. Cereals were stored so as to alleviate the losses caused by insects. Gradually, the science of preserving meat products evolved, so that it was possible to slaughter animals and not be compelled to eat the meat within a twenty-four to forty-eight hour period. c. Snow and Ice. Snow and ice have been used for many centuries to preserve meat products. Alexander the Great used snow to preserve food, and also used it to cool his wine. The Romans used snow to pack prawns (resembling shrimps) and other perishable foods. Chaff was often mixed with the snow to slow down the rate of melting. Such a mixture of snow and chaff was obviously unsanitary because microorganisms are not destroyed in such a mixture. d. Salt. Salt has been used as a preservative since the beginning of recorded history. It was used in China as early as 1200 B.C. The Jews were early users of salt to preserve meat products. This would seem natural because of their access to the supply of salt from the Dead Sea. Salt-fish was a common commodity in Ancient Greece. The Romans were schooled in the use of salt as a preservative by the Greeks. The Romans used it in preserving fish and pork products. MD

7 e. The Roman Food Supply System. The Romans became very adept at curing and preserving meat products. They established slaughter facilities and meat shops that were much superior to those found in any other part of the world. Julius Caesar was able to supply his legions with meat and other food products because of the efficient manner in which food was processed, stored, and distributed in Rome. It is presumed that the conquest of Gaul was made possible by the logistical advantage held by Julius Caesar. The Romans were able to engage in sustained combat, whereas the Gauls had to disperse in small units after each engagement to obtain food supplies. A system of supply such as the Gauls had did not enable them to face the Romans in sustained combat. f. Honey. The Greeks and Romans used honey as a preservative. Honey was also used in combination with vinegar and other ingredients. Honey, because of its high sugar content, inhibited the growth of microorganisms, and imparted a desirable taste. Honey, combined with vinegar, salt, and other ingredients, resulted in a sweet pickling solution resembling our sweet pickling process used today DEFINITIONS a. Microbiology. Microbiology is the study of microscopic organisms including bacteria, rickettsiae, viruses, yeasts, molds, and protozoa. These organisms are too small to be seen without the assistance of a microscope. They are also known as microbes or microorganisms. One must have an understanding of basic microbiology in order to take advantage of the beneficial actions of microorganisms and to counter their undesirable effects. The preservation and curing of meats is nothing more than the commercial adaptation of the knowledge of the nature, life, and actions of microorganisms gained in the classroom and the laboratory. The microorganisms that cause the problems in food spoilage are bacteria, yeasts, and molds. b. Bacteriology. Bacteriology is the study of bacteria, and more specifically, the study of the chemical and biological properties of bacteria. c. Virology. Virology is the study of viruses. d. Mycology. Mycology is the study of fungi, including yeasts, and molds. e. The Potential of Hydrogen (ph). The potential of hydrogen (ph) is a mathematical expression of the degree of acidity or alkalinity, ranging from 0 to 14, with ph 7 as the neutral point. A ph below seven, indicates acidity and above seven, alkalinity. Therefore, the lower the ph below 7, the greater the acidity; the higher the ph above 7, the greater the alkalinity. f. Pathogenic. Pathogenic is a term meaning disease producing. MD

8 g. Organoleptic. Organoleptic (sensory evaluation) relates to or involves the use of the sense organs in the subjective testing (flavor, odor, appearance) of food products MICROORGANISMS Microorganisms are living things so small that they can be seen individually only with the aid of a microscope. They are widely distributed in nature and are responsible for many physical and chemical changes of importance to plants, animals, and humans. a. Pathogenic and Nonpathogenic. Most students understand that not all microorganisms are harmful or pathogenic (disease producing), but they do not fully appreciate the fact that microorganisms make possible the continuing existence of plants and animals on our earth. In addition, many different kinds of microorganisms are used by industry to manufacture products of great value to a man such as antibiotics (penicillin) and various food products, including cheese, bread, and wine. However, the activities of nonpathogenic microorganisms are not always desirable. Food products may be spoiled, fabrics and fibers may be rotted, and fermentation processes may be upset by nonpathogenic but undesirable microorganisms. b. More Research Needed. We are interested in microorganisms because of their disease producing potential and because they are capable of causing both good and bad physical and chemical changes in food, clothing, and the general environment. We are also interested in ways and means of controlling undesirable organisms and utilizing efficiently the activities of those that are beneficial. A study of the activities and means of controlling microorganisms must be based upon the knowledge of their nature, life, and actions. c. Culturing Microorganisms. Microorganisms can be cultured (grown) for study in a specially prepared medium. Various activities or procedures have been developed in order to grow and isolate certain species of microorganisms. Microorganisms need nutrients in order to live and grow. The food source used in laboratories for growing microorganisms is called a culture medium. This medium includes an organic carbon source, a nitrogen source, inorganic minerals, and any other nutrients needed. The required nutrients are usually specific nutrients based on the physiological needs of the particular species of organism being cultured; i.e., certain amino acids, purines, pyrimidines, and vitamins. For many bacteria, a single compound (such as amino acid) may serve as an energy source, carbon source, and nitrogen source. Others require a separate compound for each. (1) Liquid or solidified culturing media. These culturing media must be moist since nutrients can only enter the microorganisms by virtue of diffusion or osmosis. The culturing material can be either liquid media or solidified media. Liquid culture media are referred to as nutrient broth and are made and kept in test tubes, while solidified media are made and kept in either test tubes or petri dishes. Agar is added to nutrient broth to solidify the media. Agar is a polysaccharide extract obtained MD

9 from seaweed and is uniquely suitable for bacterial cultivation because of its general resistance to bacterial action and its property of dissolving at 212ºF (100ºC), but not gelling until cooled below 113ºF (45ºC). It is heated to a liquid, poured either into test tubes or petri dishes and allowed to cool. When the agar media cools and gels, it is ready to have microorganisms placed on it (inoculated). Also, bacteria can be suspended in the agar while it is still a liquid. Both nutrient broth and nutrient agar media may or may not have special nutrients added, depending upon the needs of the species of microorganisms that are to be cultured. (2) In test tubes. When agar is placed in test tubes, the contents are allowed to either gel in an upright position or are slanted to create a larger surface area. The former are called "deeps" and are usually inoculated by placing microorganisms on a needle and stabbing the media. The latter are called "slants" and are inoculated by smearing organisms across the surface of the agar. (3) In petri dishes (plating). Plating is the process of inoculating microorganisms on the surface of the agar media in petri dishes. This method is most often used to grow large populations of organism colonies for further study. After the agar plates have been inoculated, they are kept at temperatures optimum for the growth of the organisms. The act of establishment, growth, and multiplication of microorganisms at a specific temperature for a given time is termed incubation. Following the establishment of organisms on agar media, the colonies and media together are termed a culture EARLY DEVELOPMENTS IN MICROBIOLOGY a. Leeuwenhoek. Anton Van Leeuwenhoek was the first man to see microorganisms. He ground lenses as a hobby and made his discovery while observing a drop of water. Seeing many tiny organisms moving rapidly under the lens, he called them "animalcules" (little animals). He is credited with inventing the microscope, and his invention made possible the development of the science of microbiology. Leeuwenhoek continued to work with microscopes until his death in 1723 and made many important discoveries. He observed bacteria on houseflies; he saw bacteria in the excreta of man and animals; and he discovered the presence of bacteria in the human mouth. b. Koch. Robert Koch was another important worker in the field of microbiology. He proved that bacteria cause disease by discovering the causative organisms of anthrax, cholera, and tuberculosis. c. Pasteur. Louis Pasteur is credited with being the founder and originator of the science of microbiology. He immunized sheep against anthrax and introduced a treatment for the prevention of rabies. Our pasteurization process for milk and other foods also bears his name. MD

10 d. Others. There have been many important contributions to microbiology and many esteemed workers in this field; however, Leeuwenhoek, Koch, and Pasteur are the individuals who were most important in establishing microbiology as a science CLASSIFICATION OF ORGANISMS a. Animal. Most animal cells have flexible outer walls, and therefore, they are usually able to move independently. b. Plant. Plant cells usually have rigid outer walls of cellulose, which surround the living protoplasm within the cell and maintain the cell shape. Unlike animals, most plants are either attached to one spot by their roots or are transported by their environment, as, for example, the liquids in which they are suspended. Plants get their energy from the process known as photosynthesis, with chlorophyll acting as a catalyst. c. Protista. The only basis for an organism being classified as a member of the kingdom Protista is its simple biological organization, characterized by a lack of extensive tissue formation. However, some organisms have a tendency to form tissues, and it is difficult to classify these organisms with great certainty. The kingdom includes both microscopic unicellular organisms and very large multicellular forms. Included in the kingdom Protista are all bacteria, fungi, rickettsiae, viruses, and protozoa. (1) Fungi. Fungi are organisms that are not photosynthetic. They are multicellular, consisting of long filaments. (2) Bacteria. Bacteria are one-celled microorganisms. They differ in shape from ball-or round-shaped, to rod-shaped, to curved or coil-shaped. Shape is an important characteristic of bacteria because it plays a key role in laboratory identification. The ball- or round-shaped bacteria are called cocci; the rod-shaped are known as bacilli; and we call the curved or coil-shaped ones spirochetes. (3) Rickettsiae. These are bacteria that can survive only in living cells. Unlike most infectious agents, rickettsiae require an intermediate host or vector in order to be transmitted from one host to another. Some of the common vectors include fleas, lice, ticks, and mites. Rickettsiae are smaller than most other bacteria. They may appear as cocci, diplococci (cocci in pairs), or short bacilli. (4) Viruses. A virus is a noncellular, submicroscopic particle that lives and reproduces in living cells. Like bacteria, viruses have many sizes and shapes that are significant factors in their laboratory identification. In size, the virus is the smallest of all infectious agents and must be viewed with an electronic microscope. Filtration is the method usually employed to separate bacteria from viruses, which may be present in animal or plant fluid. Filters catch the bacteria but let the viruses pass through. (5) Protozoa. A protozoan is a microscopic, unicellular organism without chlorophyll. They (protozoa) get their energy from organic matter. MD

11 1-6. SIZE RELATIONSHIP Special units of measurement are used to state the size of microorganisms. Size is usually measured in microns and the symbol for micron is µ, the Greek letter mu. A micron is equivalent to 1/1000th of a millimeter or 1/25,400th of an inch. To give you a better idea of the size of a micron, the diameter of a metal pinhead is about 2000 microns. The range in size of bacteria is wide. Some cells are so small that they can barely be seen by the best compound light microscopes whereas others are large enough to be almost visible with the unaided eye. Among the ball- or round-shaped bacteria, the diameter varies from 0.5 to 2.5 microns. The variations are much greater in the rod-like species--from 0.2 to 2.0 microns in width and from 1 to 15 microns in length. Most of the bacteria of importance in foods range from 2 to 10 microns in length and 0.5 to 2.5 microns in diameter. The size of viruses is much smaller, the largest being about 0.2 micron and the smallest about micron BACTERIA MORPHOLOGY AND PHYSIOLOGY a. Definition. Bacteria are microscopic, unicellular microorganisms, containing no well-defined nucleus. Most bacteria are devoid of chlorophyll and reproduce asexually by division (binary fission). (1) The cell (figure 1-1) is the structural unit of all living organisms. The typical cell is composed of cytoplasm surrounded by a cell wall, and there is a nucleus near the center of the cell. Because of their small size and controversial nature, there is no generally accepted opinion that the bacterial cell has a nucleus. The outer part of the bacterial cell is made up of three structures we call the cell wall, the cytoplasmic membrane, and the capsule. The cell wall limits the volume occupied by the cytoplasm and gives shape and rigidity to the cell. Inside the cell wall is the cytoplasmic membrane that surrounds the cytoplasm. Figure 1-1. Bacterial cell structure. MD

12 (2) The capsule is a gummy, jelly-like layer that surrounds most bacteria and varies in thickness. This capsule offers the cell some protection against adverse conditions, including drying. The capsule is the source of slime on beef carcasses. (3) Cytoplasm is the internal environment of the cell, excluding the nucleus. This is part of the actual living materials of the cell and the physical and chemical changes that occur here produce life. A highly specialized structure in the cytoplasm is the site of respiration of the cell. The cytoplasm is usually a clear, somewhat viscous substance consisting of a complex mixture of proteins, fats, oils, carbohydrates, minerals, and water. (4) The nucleus or aggregates of nuclear material are in the central portion of the cell. They transmit hereditary traits and contain the genetic controlling materials. (5) Flagella are long, fine thread-like filaments attached to the cell in various locations and they give movement or locomotion to the cell. Not all bacteria possess flagella. (6) Endospores are produced by some bacilli (rod-shaped bacteria). The endospore enables the bacterial cell to remain viable for long periods of time. It is much more resistant to drying and other adverse conditions than the vegetative cells of the species. Some bacterial spores are so resistant that they will live 20 years or more on dry splinters of wood. They will grow after being subjected to strong disinfectant solutions, and will survive for an hour in boiling water or a hot oven. The biologic significance of endospores is not known, but because of their great resistance to heat, drying, and chemicals, some bacteriologists argue that spores are produced to permit survival under unfavorable conditions. b. Shape and Size. Bacteria are spheroid, rod, or spiral in shape (figure 1-2). Spheroid (or round-shaped) bacteria are called cocci (singular: coccus), rod-shaped bacteria are called bacilli (singular: bacillus), and spiral-shaped bacteria are called spirilla (singular: spirillum). See figure 1-2. The organisms causing food fermentation, the soil bacteria, and most of the rot-producing bacteria are in the bacilli classification. The extreme smallness of bacteria may be emphasized by the fact that 400 million bacteria would occupy the volume of a grain of sugar. Bacteria are commonly magnified about 1000 times for observation in the laboratory. A man magnified to the same extent would be over a mile high and 500 yards wide. c. Arrangement. Cells possessing a well-developed slime layer, or capsule, tend to cling together, but cells without a capsule exist as single cells. Bacilli and spirilla are arranged in chains, in irregular masses, or they may exist as single cells. Cocci may exist as a diplococcus (paired), streptococcus (pairs or chains), staphylococcus (clusters), tetracoccus (square clusters of four), or sarcina (cube clusters of eight with formation of yellow or orange pigment). See figure 1-2. Bacteria often develop colonies that are large enough to be seen without the aid of a microscope. These colonies are simply masses of bacterial cells that may develop from a single vegetative cell, a single MD

13 spore, or a clump of cells or spores. Each cell functions as an independent unit. Colonies of a bacterial species will generally exhibit a distinctive and characteristic form and color. Figure 1-2. Shapes and arrangements of bacteria. MD

14 d. Reproduction. Most bacteria reproduce by binary fission. One organism divides to form two new organisms. The time required for fission will vary from eight minutes to as long as six hours. If all cells of each succeeding generation divided every twenty minutes, then one bacterial cell would result in sixty-five billion cells in twelve hours. e. Optimum Growth Conditions. Bacteria generally have a poorer tolerance for salt and sugar than most other microorganisms. Most species of bacteria have an optimum ph range between ph 6.0 to ph 8.5. Bacteria generally will not grow in as wide a range of moisture conditions and temperatures as will yeasts and molds MICROORGANISMS SMALLER THAN BACTERIA There are two groups of microorganisms smaller in size than most bacteria. They are of little importance as spoilage organisms but are of considerable importance as pathogenic organisms in food. a. Rickettsiae. Rickettsiae are smaller in size than other bacteria but are larger than viruses. Rickettsiae cause such diseases as typhus, Rocky Mountain spotted fever, and Q fever. Q fever is now considered to be one of the major milk-borne diseases in this country. b. Viruses. Viruses are our smallest group. They cause many diseases in animals, humans, and plants. A bacteriophage is a type of virus that destroys bacteria. Bacteriophages are very important in the brewing industry and dairy industry, since they will destroy beneficial bacteria that are essential to the fermentation and souring process MOLDS Molds (division Eumycetes) are multicellular microorganisms that form filamentous branching growths known as mycelia (singular, mycelium). See figure 1-3. Figure 1-3. Molds. MD

15 Individual mold cells have rigid walls surrounding the protoplasm. The cells are often cylindrical, and they vary greatly in size. Molds can be seen with the naked eye when they grow in sufficient quantities. Molds multiply by spore formation. They are usually found growing on solid substances such as wood, paper, cloth, leather, meat, fruits, vegetables, and many other substances. Molds have definite colors such as white, black, and green. They can grow in foods containing high percentages of sugar and salt. Molds are capable of growing in a ph range of 2.0 to 8.5. Molds can grow at a temperature as low as 15ºF (-9.45ºC), and a moisture content as low as 6 percent. They produce large numbers of spores that are light in weight and are easily transported by air currents. Molds are responsible for a wide variety of losses in food products, but they are also desirable in several industrial processes (penicillin production) and are responsible for the appearance and flavor of cheeses such as Roquefort and Bleu YEASTS Yeasts (figure 1-4) are unicellular organisms that are rather large when compared to bacteria. Like the molds, the yeasts belong to the division Eumycetes. Yeast cells are spherical, elliptical, or cylindrical. The protoplasm of a yeast cell is enclosed by a cell wall and cytoplasmic membrane. It contains a nucleus but there are no flagella present for locomotion. They reproduce by budding and have the ability to ferment sugars and to produce alcohol and carbon dioxide. They can grow in higher concentrations of salt and sugar than bacteria are able to tolerate. They prefer an acid medium of ph 5.5 to 6.5. Yeasts need more moisture (15 percent) than molds but require less moisture than bacteria. Yeasts are capable of growing at lower temperatures than most species of bacteria. Generally speaking, yeasts occupy a position intermediate in range between molds and bacteria relative to their tolerance for extremes of sugar, salt, acid, moisture, and temperature. Yeasts are important in bread making and the manufacture of alcohol. Figure 1-4. Yeasts. MD

16 1-11. NUTRITION AND METABOLISM OF MICROORGANISMS Microorganisms require food for energy and for growth. Food is required for energy for synthetic activities, for moving, and to maintain the living state. Food for growth is required to manufacture protoplasm. Microorganisms must have their food in solution in order for it to pass through their cell membranes. If they are not growing in a liquid medium, then the microorganisms will secrete extracellular enzymes that will liquefy their food ENZYMES Enzymes are organic substances that cause chemical reactions without being consumed in the reaction; therefore, they are called organic catalysts. They are produced by living cells and then become independent of the cells. They are incredibly active; e.g., one molecule of the yeast enzyme can catalyze the conversion of an inconceivably large number of sugar molecules into ethyl alcohol; and they are very specific, that is, they act upon only one substance. Extracellular enzymes are formed inside the cell and diffuse through the cell wall to cause their action. These enzymes bring about digestion in the human body and putrefaction (decomposition) in fruits, vegetables, and meats. Intracellular enzymes are formed in and remain within the cell. a. Characteristics. Enzymes are usually named according to the kind of substances upon which they act or the kind of chemical reaction they produce. They are suffixed "ase" as in the case of "carbohydrase," an enzyme that acts upon carbohydrate. As we have already stated, enzymes are specific; thus, maltase, a carbohydrase, is the specific enzyme that converts maltose into glucose. b. Effect of Temperature. Within certain limits, the speed of reactions caused by enzymes is doubled for each 18ºF or 10ºC rise in temperature. A 50 percent reduction in enzymatic action will result when the temperature is lowered 18ºF or 10ºC. This is referred to as van't Hoff's law or temperature quotient (known as Q10). Activity is best at temperatures between 32º-104ºF (0º - 40ºC). c. Example of Enzyme Action. As mentioned above, enzymes can initiate a considerable amount of activity. They initiate reactions without being consumed. For instance, the enzyme invertase (which changes sucrose into dextrose and levulose) will invert one million times its own weight of sugar and will be capable of more activity. d. Similarity to Living Cells. Enzymes possess some of the properties of living cells. Living cells are destroyed by high temperatures. Enzymes are inactivated by high temperatures and most of them are destroyed by a temperature of 165ºF (74ºC). Living cells have optimum temperatures for their functions, and enzymes have optimum temperatures for their actions. The actions of living cells are slowed at low temperatures; the actions of enzymes are retarded by low temperatures. Enzymes and living cells both require certain ph limits for their optimum actions. MD

17 e. Use of Enzymes in Cheesemaking. In order to make cheese, the casein of the milk must be coagulated by natural souring or by the use of rennet, a commercial preparation of the enzyme rennin that is extracted from the calf's stomach. After the curd is coagulated, it is then acted upon by microorganisms to produce the desired flavor. f. More Examples of Enzyme Action. After death, the tissues of the animal undergo a partial autolysis by enzymes contained in the tissues. Tenderizing of the meat is accomplished by the action of these autolytic enzymes. Papain, a proteolytic enzyme of papaya, is used as a meat tenderizer. Bromelin, a similar enzyme in fresh pineapple juice, is equally effective as papain and has a more desirable odor SUMMARY a. The Action of Microorganisms. Many kinds of microorganisms are most desirable for the part they play in preserving and processing foods. On the other hand, some microorganisms attack food. Microorganisms such as bacteria, molds, and yeasts damage or destroy foods. Bacteria produce certain enzymes that have either a beneficial or a destructive effect on some foods. Bacterial growth in or on foods often is extensive enough to make the food unattractive in appearance or objectionable in some other way. Some bacteria cause food poisoning, some produce bitter flavor, and others cause foods to spoil in one way or another. Molds cause some foods to decompose. The fermentation effect of yeasts causes spoilage among many kinds of foods. b. Microorganisms and Enzymes. Microorganisms include bacteria, molds, yeasts, rickettsiae, and viruses. Enzymes are not included among the microorganisms. An enzyme is an organic catalyst INTRODUCTION Section II. MICROBIAL GROWTH Many species of bacteria increase in numbers very rapidly under ideal conditions. Many scientists believe that the rapid growth of bacterial cells is because of their large surface-to-volume ratio. In order to maintain their small size and the favorable surface-to-volume ratio, cell division must occur rapidly. Most living organisms have a cellular structure and grow by an increase in the number of cells per organism. Bacteria, however, retain their unicellular structure and growth is reflected in an increase in the number of individual cells, or organisms. Cell division of unicellular bacteria appears to follow a very similar pattern and occurs in four steps. a. Step 1. The cell nucleus divides before the cell division occurs. b. Step 2. The second step is the division of the cytoplasm into two equal parts separated by an inward growth of the cytoplasmic membrane forming a cross plate. In cylinder-shaped bacteria, this division is generally at right angles to the long axis. MD

18 c. Step 3. The third step involves formation of a cross wall which grows inwardly from the cell wall and splits the cross plate so that each of the newly formed cells has a continuous cytoplasmic membrane. The cross wall then divides, providing each daughter cell with a complete cell wall. d. Step 4. The last step in cell division is the separation of the sister cells. The cells of many bacterial species separate shortly after the cross wall is formed. Such bacteria generally appear as single cells and form smooth colonies on solid culture media. Other species have a cell wall which does not tend to split between cells and which withstands the stresses induced by continued growth. Such bacteria do not separate easily and form chains or other groupings such as sheets, packets, or irregular clumps. Colonies from rod-shaped bacteria forming long chains generally appear rough or wrinkled because of the buckling of the chains as they meet resistance to their continued elongation CELL GROWTH If conditions are favorable, cell division is normally followed by a period of cell growth or enlargement. The cell grows to its original size through absorption of water and food and through manufacture of protoplasm. This growth is at right angles to the plane in which the division takes place. Quite naturally, during such a period of cell division and growth in a culture, the new cells are only half as long as the mature cells. In some spherical or coccus forms, the new daughter cells are hemispheres when first formed and are then restored to full spheres through growth. In some short rod cells, the cell elongates first and then divides GROWTH CURVE Growth of microorganisms refers to an increase in the number of unicellular organisms. Under favorable conditions of nutrition, oxygen, ph, moisture, and temperature, some kinds of bacteria may double in number about every 20 minutes. This time interval is called generation time. Simple arithmetic shows the magnitude of the result if this rate of increase were to continue for only a few days. Fortunately, there are several factors that help control the situation. In some cases, the food supply may become depleted or the accumulation of waste products may slow the process. However, when these conditions are controlled by the continuous addition of nutrients and removal of waste products, the bacterial population always reaches a maximum before the medium in which it is growing becomes a solid mass of cells. a. Four Phases. Different species of bacteria show various shapes of growth curves depending on the generation time and the maximum population attainable under the prevailing growth conditions. The growth curve is determined by plotting the numbers of bacteria per milliliter of culture against incubation time. The counts are plotted with logarithms of numbers. The growth curve of microorganisms is composed of four phases (figure1-5). MD

19 Figure 1-5. Growth curve of microorganisms. (1) During a period of one to several hours, there is a lag phase in which there is little or no increase in cell numbers. During this phase, the cells are becoming adjusted to a new environment. (2) The log (logarithmic) phase is a period of rapid growth. The growing cells divide and continue to do so at regular intervals until maximum growth can be supported. In this phase, the microorganisms are well adjusted to their environment and are able to multiply rapidly. (3) The stationary phase is one in which the population remains unchanged. The rate of reproduction equals the death rate in this phase. (4) The decline (death) phase occurs when the death rate exceeds the rate of reproduction. b. Ways to Lengthen the Lag Phase. Growth control is important in food preservation. Prevention or delaying food spoilage is accomplished by lengthening the lag phase as much as possible. This can be done in several ways: (1) By the introduction of as few spoilage organisms as possible, that is by reducing the amount of contamination. The fewer organisms present, the longer the lag phase. (2) By avoiding the addition of actively growing organisms, which may be growing on unsanitary containers, equipment, or utensils with which food comes in contact. MD

20 (3) By control of one or more unfavorable environmental conditions such as nutrients, moisture, ph, or temperature. The greater the number of conditions that are unfavorable means the longer the delay of the beginning of cell growth. (4) By damage to organisms through processing methods such as heat or irradiation FACTORS INFLUENCING GROWTH OF MICROORGANISM There are several factors that influence the growth of microorganisms: nutrition, oxygen, ph, temperature, and moisture. Lack of food retards bacterial growth, and growth is favored by a sufficient quantity of the proper kind of food. Moisture is required to carry foods in solution into the cell, to carry wastes in solution away from the cell, and to maintain the moisture content of the cytoplasm. Temperature has a profound influence on the growth rate of microorganisms. Microorganisms subjected to adverse temperatures are either destroyed or are not able to multiply. The optimum temperature of a microorganism is the temperature that provides for the most rapid growth of that microorganism. The ph of the medium in which microorganisms grow exerts a considerable influence on their rate of growth. All microorganisms have an optimum ph at which they grow best. Most species of bacteria have an optimum ph between 6.0 and 8.5. Molds will grow in a ph range between 2.0 to 8.5. Yeasts have an optimum ph range from 5.5 to NUTRITION Nutrients or foods are substances which are outside the cell and which, upon entering a cell after passing across the cell membrane, can be used by the cell for building material or for obtaining energy. Food requirements of bacteria show great variations from species to species. Some organisms can obtain all their food requirements from inorganic matter while others need many complex organic compounds. Although any one species may be able to use only a small number of materials as sources of food, bacteria as a group are able to utilize all the naturally occurring organic compounds as well as many inorganic substances. a. Requirements. Bacteria require foods for the same purposes, as do other forms of life, namely, as sources of material for cellular synthesis and for energy in order to perform these synthetic processes. Requirements include carbohydrates (sugar, starches, and celluloses), a source of nitrogen, vitamins, water, and a source of energy. b. Sources. The majority of bacteria species use naturally occurring organic materials such as carbohydrates, proteins, fats, and so forth, not only as sources of carbon, hydrogen, oxygen, and nitrogen but also for the energy needed to synthesize these materials into protoplasm (that material referred to as the physical basis of life and which is common to all living cells). The requirements of most bacteria for inorganic materials can be satisfied by salts containing sodium, potassium, calcium, magnesium, MD

21 iron, chlorine, sulphur, and phosphorus. More or less complex sources of carbon, hydrogen, oxygen, and nitrogen may be obtained from organic sources, inorganic compounds, or from the elemental forms of these substances. c. Deficiencies. Many bacteria have deficiencies in their synthetic abilities that must be overcome before growth can follow. The vitamin B complex is the most common source for this compound requirement. Other compounds often found necessary for maximum growth are the amino acids, which are the building blocks of protein. d. Methods. There are two primary methods by which bacterial species secure their food material. On this basis, bacteria are classified as follows: (1) Saprophytes. These bacterial species get their food materials from the dead bodies and waste materials of plants and animals. They also make some use of inorganic compounds such as ammonia, nitrates, chlorides, phosphates, and sulfates. Most of the saprophytes are harmless to man. Two exceptions are Clostridium botulinum and Clostridium tetani. (2) Parasites. These bacterial species secure their nourishment from the cells of plants or animals with which they intimately live. The parasitic bacteria are mostly harmful to man OXYGEN REQUIREMENTS Few processes have had such a wide variety of definitions as respiration. We normally think of respiration as being synonymous with breathing, but a broader meaning is necessary to identify bacterial respiration, which is accomplished without a breathing apparatus. Bacterial respiration is any chemical reaction whereby energy is released for life processes. The reactions are those that transform energy for the cell. Energy transformations can take place under aerobic or anaerobic conditions or combinations of these conditions. On the basis of their processes of respiration, bacteria are classified as follows: a. Aerobic bacteria require the presence of free or atmospheric oxygen for growth. b. Anaerobic bacteria do not require the presence of oxygen for growth and will grow better in its absence. c. Facultative bacteria will grow either with or without the presence of oxygen. d. Microaerophilic bacteria require a definite but reduced amount of free oxygen for growth. MD

22 1-20. ACIDITY AND ALKALINITY a. Foods and the ph Scale. Most bacteria are sensitive to changes in the acidity or alkalinity of their environment. Acidity and alkalinity are determined by the concentration of the hydrogen ion. The symbol for the logarithm of the reciprocal of the hydrogen ion concentration is ph. The ph scale extends from 0 to 14 with a ph of 7.0 being neutral. A solution is acidic because of an excess of positively charged hydrogen ions or it is alkaline because of an excess of negatively charged hydroxyl ions. Numbers below 7.0 on the ph scale indicate an acid condition with 1.0 or less being the most acid. Numbers above 7.0 on the ph scale indicate alkalinity with 14.0 being the most alkaline. (1) Foods have been categorized according to their ph as follows: High-acid foods ph below 3.7 Acid foods ph Medium-acid foods ph Low- or non-acid foods ph over 5.3 (2) The approximate ph values for selected foods are listed below. The ph values are only approximations, since there is considerable variation in the ph of some foods and the ph of foods and the ph of food products can change during ripening, processing, or spoilage. Lemons Apples Tomatoes Spinach Milk Shrimp b. The Optimum ph Range for Bacteria. Bacteria have an optimum range in which they grow and function best. For many, this is near the neutral point or in a ph range of from 6.0 to 8.5. Some bacteria can live in or even require a very acid environment such as a ph of 3.0 or less. Some bacteria prefer an alkaline ph of 8.0 or higher. An important phase of bacterial physiology is the determination of the optimum, minimum, and maximum ph for organisms being investigated. c. The Optimum ph Range for Molds and Yeasts. You will recall that molds can grow over a wider range of ph values than can yeasts, as evidenced by the difference in range of ph values: molds from 2.0 to 8.5, and yeasts from 5.5 to 6.5. Bacterial growth is favored by a near neutral ph. An example of the importance of knowledge about ph values and food microbiology is that the food organism Clostridium botulinum prefers a near neutral ph for growth and its growth is inhibited by increased acidity. MD

23 1-21. TEMPERATURE a. Growth Range. Bacteria as a group are able to grow over a range of about 175-Fahrenheit degrees (F). This is a relatively narrow range when one considers the coldest arctic to the hottest volcano. However, it is a wide range when compared to the changes acceptable in man's body temperature. No one bacterial species can grow over the entire 175-degree range but some are capable of growing over about one-half of it; that is, from near 32ºF (0ºC) to 104º to 113ºF (40º to 45ºC). Each organism can grow only within a growth temperature range characteristic of the species. (1) Minimum growth temperature. The lowest temperature at which growth occurs is the minimum growth temperature. This is difficult to determine because physiological activities gradually decrease with the temperature until they can no longer be detected by ordinary means. The generation time may be increased from minutes to days or weeks. (2) Maximum growth temperature. The highest temperature at which growth can take place is the maximum growth temperature. This is more readily defined and can occasionally be determined within a Fahrenheit degree. (3) Optimum growth temperature. The temperature at which most rapid multiplication occurs is the optimum growth temperature. It is the point at which the generation time is the shortest. This may also be difficult to define because it may be altered by both chemical and physical factors. There is no one temperature that is optimal for all activities of the cell. b. Three General Groups. Bacteria have been divided into three large groups on the basis of their growth temperatures. These groups are not sharply defined and the distinctions are arbitrary, but the classification serves some practical purposes. The groups are called thermophiles, mesophiles, and psychrophiles. (1) Thermophiles. Thermophilic bacteria are interesting because they prefer temperatures intolerable to most forms of animal life. Water at 113ºF (45ºC) is hot to the touch--hotter than the ordinary bath. Thermophilic bacteria grow best at temperatures above 113ºF (45ºC). We arbitrarily say that the temperature range for this bacterial group is from 113º to 140ºF (45º to 60ºC), although they may grow anywhere within the range of 104º to 176ºF (40º to 80ºC). Thermophiles are particularly troublesome in the dairy industry since they may grow most rapidly at pasteurization temperatures. They may also cause spoilage of canned foods, which are stored at elevated temperatures because the spores of some thermophilic bacteria are extremely resistant to heat and survive the ordinary canning processes. (2) Mesophiles. Mesophilic bacteria are the ones that are most common, and they grow best at the moderate temperatures ranging from 65ºto 105ºF (18º to 41ºC). The minimum and maximum growth temperatures vary, for the most part, within the range 50º to 125ºF (10º to 52ºC). MD

24 (3) Psychrophiles. Psychrophilic bacteria have a temperature range from 32º to 50ºF (0º to 10ºC) with some species multiplying at temperatures as low as 23ºF (- 5ºC) in solutions that do not freeze solid (brine and sugar solutions). Organisms that grow well at 32º to 41ºF (0º to 5ºC) are often encountered as causes of spoilage in refrigerated foods, but their rate of growth at such temperatures is usually slow and spoilage is not apparent unless storage is greatly prolonged. c. Narrow Growth Range. The term microphile is used to designate bacteria that have a narrow range of temperature for growth. By that, we mean the maximum and minimum temperatures are relatively close together. Most microphiles are mesophilic, with a temperature range between 86º to 104ºF (30º to 40ºC). d. Characteristics. Generally speaking, the optimum temperature is much closer to the maximum temperature than to the minimum temperature. When bacteria are submitted to temperatures a little above the maximum or a little below the minimum, they are not necessarily killed but may enter a relatively dormant state EFFECT OF HEAT a. Mesophilic Bacteria. Most mesophilic bacteria are killed if heated in a liquid medium to 122º to 149ºF (50º to 65ºC) for a few minutes, say 10 minutes. Some bacteria (those classified in the genera Bacillus and Clostridium) may form spores, which are highly resistant to boiling. Boiling for minutes to hours does not destroy these bacteria. The spores of some species can withstand temperatures above the boiling point of water for a half hour or longer. b. Thermophilic Bacteria. Thermophilic bacteria, which do not form spores and which are capable of withstanding temperatures of 140º to 158ºF (60º to 70ºC) for the time required to pasteurize milk, are called thermoduric. These cause considerable trouble in the milk-processing industry. c. Resistance Factors. In determining the heat resistance of bacteria, the following factors are considered jointly: (1) The temperature. (2) The length of time during which the bacteria are exposed to the heat. (3) Whether the bacteria are heated in a moist or a dry condition. (4) The ph concentration of the medium in which the bacteria are heated. (5) The other characteristics of the medium. For example, bacteria are killed at a lower temperature in water than in cream. MD